Electronic mail (e-mail) provides fast, sometimes apparently instantaneous, communication of many types of information. For this reason, e-mail has become widely used for both business and personal communications.
Unfortunately, e-mails can sometimes become lost or delayed. An e-mail message could be delayed or lost for any of a number of reasons, including overload, failure (e.g., disk crash), or upgrade of a server along the end-to-end, store-and-forward path from the sender to the recipient. Overload or failure is sometimes triggered by a burst in the volume of e-mail messages because of spam or the spread of a virus. Furthermore, the widespread use of spam filters also contributes to e-mail delivery problems such as by sometimes quarantining legitimate e-mail messages, delaying the e-mail until the intended recipient recognizes that the e-mail was in-fact received.
The most widely used e-mail protocol, the Simple Mail Transfer Protocol (SMTP), includes provisions for an e-mail server to automatically generate delivery delay messages when it has held an e-mail in a queue for an extended period. While such messages can improve the experience for a user, they may not be effective in all cases. For example, some corporations do not allow any such messages to be generated to protect the privacy of the corporation (e.g., it prevents an entity from verifying if an e-mail address is valid) or prevents such messages from being generated or received in at least some cases, such as when the e-mail is regarded as spam. Further, such messages cannot be generated for e-mails that experience delivery problems before reaching an e-mail server.